1. Field of the Invention
The present invention relates, generally to two-speed electric motors and, more specifically, to a two-speed electric motor having a convex shaped high-speed brush.
2. Description of the Related Art
Direct current (DC) motors are common and used in a wide variety of applications as a source of motive power. DC motors operate by causing an armature to rotate in a magnetic field to produce torque. In one form of DC motor, a magnetic field is developed within the motor case surrounding an armature that consists of a series of wire wound coils, or windings. Each end of each of the armature windings is electrically connected to one segment of a commutator. The commutator is a series of electrically separate cylindrical segments that are placed about the armature shaft. Brushes are set in contact with the commutator to deliver voltage and current flow to the armature windings. The current applied to the armature windings produces an electromagnetic field that interacts with the magnetic field within the motor case. This interaction drives the armature through a portion of its angular rotation. Due to the segmented commutator, the voltage polarity and the current flow will reverse in each of the armature windings at every half cycle of their rotation. This causes the armature to continue to rotate, which in turn produces a torque output on the armature shaft.
Many DC motor applications call for the motor to produce two output speeds. A two-speed DC motor has two sets of brushes, one for low speed and one for high speed. More precisely, it is common practice to employ one common brush that operatively interacts with one low-speed brush and with one separate high-speed brush to provide two different motor speeds. The relative location of the brushes about the commutator determines how many armature windings are connected in the circuit to produce the electromagnetic field to turn the armature. In typical two-speed DC motor construction, the common and low-speed brushes are placed 180 degrees apart. This causes the greatest number of available windings to be electrically charged during the rotation of the armature.
To achieve a high-speed output, the high-speed brush is physically located angularly closer about the commutator to the common brush. Therefore, as the armature turns there are fewer armature windings connected between the common and high-speed brush, than between the common and low-speed brush. This causes the armature to rotate faster when the high-speed brush provides the voltage and current. During the initial wear-in period of conventional two-speed DC motors, the seating of the brushes, and particularly the high-speed brush, causes the motor speed to increase. This is somewhat noticeable when the motor is operating at low speed, but becomes undesirably distinct at the high speed setting. It is common to radially offset the high-speed brush in an attempt to minimize this problem. However, a radial offset of the high-speed brush causes the brush to drag on the commutator and slow the armature during wear-in causing a like undesirable reduction in motor speed.
Furthermore, when running the two-speed DC motor in the high-speed mode, the initially slowed operation during the wear-in period may cause problems for the system it is employed in. Many mechanically complex systems, when newly manufactured, are stiff and require their own break-in period to overcome the initial tightness of the various joints and pivot points and to allow for the provided lubrication to reach all portions of the mechanism.
For example, windshield wiper systems commonly found in motor vehicles most often employ two-speed DC motors as their source of operative power. Conventional motor vehicle windshield wiper systems are mechanically complex with a number of rotating shafts, linkage arms, and pivot points. Additionally, the two-speed DC motors used in windshield wiper systems employ gear reduction assemblies to transfer output torque from the motor to the wiper system. All of the mechanical interaction between the parts of the wiper system and the gear reduction assembly of the DC motor, in addition to the slowed armature speed due to the high-speed brush wear-in cause the high-speed mode in many newly manufactured wiper systems to be nearly as slow as the low speed setting.
Eventually, the wiper system components loosen and the DC motor increases in speed as the break-in and wear-in periods complete. This generally brings the operating speed of the high-speed mode up into a desired speed range. However, this is an undesirably long process often requiring more than an hour of run time at the high-speed setting. Furthermore, when the motor speed increases and the wiper system loosens, the resultant speed change in the high-speed setting may be excessively high. For automotive manufacturers, this results in a high number of new car owner complaints regarding the high-speed operation of the wiper system.
Accordingly, there remains a need in the related art for a DC motor with an improved high-speed brush that does not increase or maintain its high-speed output as the high-speed brush wears in. Further, a need exists for a DC motor with an improved high-speed brush that has an initially faster high-speed output so that it may be employed in a newly manufactured mechanical system to overcome any speed reduction caused by the stiffness of the new system. Finally, there remains a need for a DC motor with an improved high-speed brush that subsequently reduces its initially faster high-speed output to a desired level after a certain period of time so that it may be employed in a newly manufactured mechanical system, to first overcome any speed reduction caused by the stiffness of the new system, and to second slow as the system loosens so that no apparent change in the system speed occurs.